The present invention relates to a motor, a commutator provided in the motor, a short-circuit member provided in the commutator, and a method for manufacturing the commutator.
When a motor in which electric current is supplied through brushes that slide on segments of a commutator is driven, spark discharge can occur between the commutator and the brushes. Spark discharge is one of the factors that shorten the life of the brushes. Thus, some prior art motors include a varistor, or a surge absorbing element, to prevent spark discharge. The varistor is electrically interposed between the ends of each armature coil, which is part of the motor.
For example, Japanese Laid-Open Patent Publication No. 2005-5293 discloses a motor, in which a varistor is fixed to a commutator. The commutator includes a cylindrical insulator press fitted about the rotary shaft of the motor and three segments fixed to the outer circumferential surface of the insulator. Each of the three segments is connected to an end of one of the armature coils of the motor. Brushes for supplying currents to the armature coils slide on the segments. A riser extends outward in the radial direction from one end of each segment in the longitudinal direction. An annular varistor is located on the risers. A contact piece formed at the distal end of each riser is bent, so that the varistor is tightly held between the risers and the contact pieces. This electrically connects the varistor with the segments, and the varistor is located between the ends of each armature coil.
The structure in which a varistor is interposed between the ends of each armature coil can be applied to multipolar motor such as the motor disclosed in Japanese Laid-Open Patent Publication No. 2005-137193. The motor disclosed in the publication has segments the number of which is the number of the armature coils multiplied by an integer greater than one. The ends of each armature coil are connected to a predetermined adjacent pair of the segments. The publication discloses as one example a motor having six permanent magnet and eight armature coils. Among twenty-four segments of the motor, segments of the same potential are short-circuited, and thus the number of the brushes is two.
However, when applying the structure in which a varistor is electrically interposed between the ends of each armature coil is applied to the motor disclosed in Japanese Laid-Open Patent Publication No. 2005-137193, a connection portions is needed between each end of the armature coils and the varistor (that is, between the varistor and each segment to which one of the ends of the armature coils is connected), and the number of the required connection portions corresponds to the number of the ends of the armature coils, that is, the number of the coils multiplied by two. For example, when the number of the coils is eight, sixteen connection portions are required between the ends of the armature coils and the varistor. In the case of a multipolar motor having a greater number of armature coils, the number of the connection portions increases, accordingly. This complicates the manufacturing procedure of the commutator.
The motor disclosed in Japanese Laid-Open Patent Publication No. 2005-137193 has more poles than the motor disclosed in Japanese Laid-Open Patent Publication No. 2005-5293. Thus, the motor of the publication number 2005-137193 has a greater number of contact pieces for holding the varistor with the risers than the number of contact pieces of the motor of the publication number 2005-5293. The greater the number of the contact pieces, the more complicated the process for attaching a varistor to the commutator by bending the contact pieces becomes.
As shown above, an increase in the number of connection portions and contact pieces is likely to reduce the productivity of commutators.